Extrusion axis incorrect in Blender 2.90
From what I understand, since 2.9, the default axis for extrusion is still the face normal, but if you press X or Y once, the extrusion will follow the face X or Y direction, not the global direction anymore.
Now if you press XX or YY, the followed direction will be along the global orientation. But you don't need to press Z twice to make the extrusion follow the global: As no Z was necessary in the first place to follow the face orientation, you just need to press Z once to make it follow the global.
Here is how, when you extrude then press Y or Z, the face may follow the same direction, because the face Y direction and the Z global orientation may be aligned.
Yes, a bit confusing, but the previous method was not perfect either, one X or Y meant global axis, but for example, as the default extrusion occurred along the face Z direction, to extrude along the Z global axis you needed to press Z three times.